1. Field of the Invention
The present invention relates to a harmonic characteristic measuring method and a harmonic characteristic measuring apparatus.
2. Description of the Related Art
Conventionally, it has been important to decrease harmonics by making an appropriate filter capacity, etc., in a power sending and distributing system, namely, a three-phase power system.
The n-order (n is an integer of 1, 2, . . . ) harmonic (nth harmonic) is an integral multiple of system fundamental wave frequency fs and a representative frequency of the fifth harmonic is 5.multidot.fs.
To decrease the harmonic, a harmonic level (voltage level) is predicted and a filter facility of the frequency is added to a capacitor facility, etc.
To predict the harmonic level, it is important to keep track of the harmonic characteristic downstream (load side) from the connection point of the filter facility, for example, of the three-phase power system and find its equivalent circuit (harmonic equivalent circuit), etc.
At this time, if the harmonic equivalent circuit is represented by the Norton theorem, it can be assumed to be a parallel circuit of admittance and a current source and the admittance is the most important to keep track of the characteristic.
It is described on Denkigakkai ronbunshi B, vol. 101 No. 8, p.451-p.458 (August, 1988) that when a harmonic equivalent circuit is found for the fifth harmonic of a power distribution system, the voltage and current of a fundamental wave in the system are measured and the admittance of the harmonic equivalent circuit, the current source size and phase, and the like are calculated and estimated from the measurement result.
However, to measure the harmonic characteristic of a three-phase power system by the measurement method described here, the voltage and current of the fundamental wave in the system are measured and the admittance of the targeted harmonic of the fifth harmonic, etc., the harmonic current source size and phase are estimated indirectly from the measurement result, thus it is impossible to keep track of the harmonic characteristic with good accuracy.
Thus, it is impossible to keep track of the harmonic characteristic of the three-phase power system precisely for installing an appropriate filter facility, etc., and a good decrease in the harmonic level cannot be carried out.
The present applicant already invents the following harmonic characteristic measuring method according to Japanese Patent Application No. Hei 8-310192: Currents at non-integral multiple frequencies of fundamental waves on both the upper and lower sides of measured harmonic (targeted harmonic) are injected into a harmonic injection point in a power system, admittances of equivalent circuits to the injection frequencies on both the upper and lower sides of the targeted harmonic downstream or upstream from the injection point in the system are found based on the actual measurement results of voltages at the injection frequencies at the injection point and currents at the injection frequencies flowing upstream and downstream from the injection point, and the admittance of equivalent circuit to the targeted harmonic is determined by performing interpolation processing, thereby measuring the harmonic characteristic of the targeted harmonic.
In this case, the currents at the injection frequencies are currents of so-called interharmonics of non-integral multiples of the fundamental wave frequency not originally existing in the system, the admittances of the equivalent circuits to the injection frequencies can be found with accuracy from actual measurement without receiving the effect of already existing harmonics in the system, and the result can be used to keep track of the harmonic characteristic of the targeted harmonic with accuracy.
To execute the harmonic characteristic measuring method described in Japanese Patent Application No. Hei 8-310192, it is necessary to injection interharmonic currents of two frequencies f.alpha. and f.beta. (f.alpha.&lt;n.multidot.fs&lt;f.beta.) of non-integral multiples of the system fundamental wave frequency fs sandwiching the nth targeted harmonic to be measured (frequency n.multidot.fs) into the power system.
It is possible to injection current into each phase of a three-phase power system as currents at the two frequencies f.alpha. and f.beta..
However, if an attempt is made to measure a harmonic characteristic by injecting current into the three phases, a device of a three-phase capacity becomes necessary as a current injecting device at the two frequencies f.alpha. and f.beta. and the injected current amount cannot be lessened in measuring.
To measure a harmonic characteristic in a power system for one or more harmonics by the harmonic characteristic measuring method described above, it is necessary to not only injection each interharmonic current into the power system, but also collect and process measurement current and voltage data in the power system based on the injecting; it is desired to collect and process the data automatically and rapidly.
In the harmonic characteristic measuring method described above, if an interharmonic frequency component exists in the system, it becomes noise, causing a measurement error to occur.
Therefore, hitherto, interharmonic current has been set to a comparatively large, proper level before being injected into a power system.
Thus, a large-capacity, large-scaled current injecting device becomes required for injecting interharmonic; a harmonic characteristic in a power system cannot be measured in a small injection capacity.
The following method is also possible: An interharmonic injecting point system (injection system) and a measurement point system (measurement system) are made different, a system upstream from the injection system or any branch system other than the injection system downstream from the upstream system is adopted as the measurement system, and a harmonic characteristic in the upstream system or the branch system is measured for enlarging the measurement range to the outside of the injection system.
However, as the interharmonic injecting point and the measurement point become distant from each other, the injected current measurement level lowers.
Particularly in an upstream system, the injected current measurement level furthermore lowers due to the transforming station transformer between the systems.
Thus, if a measurement point is set in an upstream system or a branch system, the noise effect becomes extremely large and a measurement error becomes large; in fact, the measurement range cannot be enlarged.